or' fix Aiit/>/iil><>h and Pyroxem groups. 



:;»;«.» 



trates the orientation observed in a typical section near the olino- 

 pinacoid, showing angite Barronnded by brown hornblende, with 

 blue am phi bole added to the latter. This orientation agrees 

 fully with the observations recorded by \)v. (i II. Williams in 

 the article preceding this, which have led him to the conclu- 

 sion that in the monosymmetric amphiboles the plane Poo of 

 the usually adopted orientation should he taken as the basal 

 plane OP. As Dr. Williams clearly shows, this orientation 

 better expresses the remarkable harmony existing between 

 these groups in regard to several physical properties as well as 

 t<> the purely crystallographical. The change has been advo- 

 cated on the latter ground by various authors and the facts 

 here presented seem to form a Btrong argument in the same 

 direction. 



In fig. 10 are presented a number of diagrams expressing 

 the range of the variations in optical orientation within the 

 two groups, the amphiboles being drawn in the new position. 

 It will he seen at a glance that in this way i\\v relationships of 

 the groups are made clear. Rieheckite has been drawn in the 

 position naturally indicated for it by analogy with aegirite ami 



10. 



IV. V. VI. 



II. 



III. 



I. Anthophvllite. IT. Glaucophane. III. Tremolite, etc. IV. Brown horn- 

 blende. V. Blue ampliibole (Silver Clift"). VI. Riebeckite (?) 



I. BSnstatite, etc. II. Spodumene. III. Diopside, etc. IV. Augite. V. 

 Augite. VI. .Egirite. 



the blue amphihole of Silver Cliff. Arfvedsonite will have 

 the position assigned to the bine amphihole or one at right 

 angles to it, according as a or c is near 6. The latter position 

 would be unique among the amphiboles. Barkevicite, if its 

 orientation is correctly given by Rosenbusch, will be near 

 glaucophane. Should it be proven for either of these minerals 

 that the axis of least elasticity lies near c then we plainly can- 

 not put that emphasis on the influence of chemical^composition 



